1. Field of the Invention
This invention relates to an optical wavelength conversion device for converting an optical fundamental wave entering the device to an optical second harmonic having a wavelength one half the wavelength of the fundamental wave. This invention particularly relates to an optical wavelength conversion device formed to exhibit low optical propagation loss and high durability.
2. Description of the Prior Art
Various attempts have heretofore been made to convert the wavelength of a laser beam to a shorter wavelength by utilization of optical second harmonic generation using a nonlinear optical material. As an optical wavelength conversion device for carrying out wavelength conversion in this manner, there has heretofore been known a bulk crystal type device as described in, for example, "Hikari Electronics No Kiso" (Fundamentals of Optoelectronics) by A. Yariv, translated by Kunio Tada et al., Maruzen K.K., pp. 200-204. However, the bulk crystal type optical wavelength conversion device has the drawback that double refraction by a crystal is utilized for satisfying the phase matching conditions, and therefore a material which exhibits large nonlinearity, but exhibits little or no double refraction, cannot be utilized.
As one of optical wavelength conversion devices that eliminate the aforesaid drawback, a fiber type device has heretofore been proposed. The fiber type optical wavelength conversion device is constituted by an optical fiber comprising a cladding and a core which is formed of a nonlinear optical material and which is filled inward of the cladding. Ah example of the fiber type optical wavelength conversion device is described in Bulletin of Micro Optics Research Group, Optical Meetings of The Japan Society of Applied Physics, Vol. 3, No. 2, pp. 28-32, Apr. 25 (1985). The fiber type optical wavelength conversion device has the advantage that phase matching between the fundamental wave and the second harmonic can be achieved easily, and therefore extensive research has been conducted in recent years on the fiber type optical wavelength conversion device. Also, as disclosed in, for example, Japanese Patent Application Nos. 61(1986)-159292 and 61(1986)-159293, there has heretofore been proposed an optical waveguide type optical wavelength conversion device comprising two substrates as cladding layers, and an optical waveguide formed of a nonlinear optical material and disposed between the two substrates. The optical waveguide type optical wavelength conversion device also has the aforesaid advantage.
However, the fiber type optical wavelength conversion device and the optical waveguide type optical wavelength conversion device are constituted so that the second harmonic is radiated into the cladding layer having a refractive index lower than the refractive index of the nonlinear optical material and phase matching is effected between the second harmonic and the fundamental wave advancing through the nonlinear optical material. Therefore, the fiber type optical wavelength conversion device and the optical waveguide type optical wavelength conversion device has the drawback that an optical propagation loss readily arises. Specifically, the second harmonic radiated from the nonlinear optical material into the cladding layer advances to an edge face of the optical wavelength conversion device by repeating total reflection. In the case where, for example, flaws or foreign substances are present on the surface of the cladding layer or a device supporting means contacts the surface of the cladding layer, the total reflection conditions are not satisfied at the surface of the cladding layer. As a result, the second harmonic escapes out of the surface of the cladding layer, or is scattered by that surface.